Fissionable nuclear fuel for nuclear reactors comprises a variety of compositions and forms of fissionable materials, including ceramic compounds of uranium, plutonium and thorium. Fuel compounds for commercial power generating reactors typically comprise oxides of uranium, plutonium and thorium, and mixtures thereof. The generally most suitable and commonly used fuel for such commercial nuclear reactors is uranium dioxide. Such commercial fuel materials can be combined with minor amounts of other ingredients, including neutron flux controlling additives such as gadolinium.
Commercially produced uranium dioxide is a fine, fairly porous powder, a form which is not suitable as such for use as fuel in commercial reactors. A number of means have been developed and used to convert powdered uranium dioxide into a form suitable for use as a fuel in power generating nuclear reactors. One commonly used technique has been to sinter appropriately sized bodies of the powdered uranium dioxide material at high temperatures to develop strong diffusion bonds between the individual power particles.
However, the sintering technique requires a preliminary compressing of the loose powder into a shaped, and self-retaining compacted body of particles of sufficient strength and integrity to survive handling and the sintering procedure. The operation of compressing fine particles into a body or coherent compact with acceptable low reject levels, and with the strength and uniformity for enduring subsequent handling and firing has been a subject of considerable concern and investigation in the nuclear fuel industry.
Conventional organic or plastic binders commonly used in powder fabrication have been considered to be unsuitable in nuclear fuel processing operations. Entrainment of any binder residues such as carbon within the sintered nuclear fuel product is unacceptable in reactor service. Moreover, the presence of any organic binder among the particles inhibits the formation during sintering of strong diffusion bonds between the particles, and adversely affects the density of the sintered product. The complete removal of binders, or their decomposition products, prior to sintering is especially difficult, and usually requires a costly additional operation in the fuel manufacture.
Accordingly, a common method has been to die press uranium dioxide powder into approximately sized "green" (unfired) compacts without the assistance of any binder. This approach however has resulted in very costly high rates of rejects and scrap material recycling because of the weakness of such green, binder-free compacts of powder.
U.S. Pat. No. 4,061,700, issued Dec. 6, 1977, to Gallivan, and assigned to the same assignee as this application, discloses a distinctive group of fugitive binders consisting of carbonate and carbamate based compositions that improved the production of sintered pellets of particulate nuclear fuel materials for nuclear reactors. The fugitive binders of this patent function without contaminating the resulting fuel product, and they permit the formation of effective bonds between sintered particles during firing without deleteriously affecting the desired porosity of the fused pellet.
The prior art techniques or means such as disclosed in U.S. Pat. No. 4,061,700, have been found wanting in some conditions and circumstances. For instance it has been observed that the fugitive binders of the aforesaid patent do not provide consistent results as to pellet strength and integrity irrespective of the blending conditions and particle characteristics of the uranium dioxide powder. Specifically the severity of agitation in blending, relative humidity and temperature, and duration of storage, as well as the uranium dioxide powder properties such as size, surface area and moisture content are all factors that apparently can detract from the uniformity of the physical attributes provided by such fugitive binders. These shortcomings are far more evident when higher rates of die pressing are applied in the compacting operation.
More effective and practical fugitive binder systems have been provided in this art for imparting improved plasticity to such particulate ceramic materials for their consolidation into coherent compacts with a minimum of rejects over a wide range of production rates, including high speed pressing with continuous rotary presses. Examples of such improved fugitive binder systems comprise the amine-type carbonate and carbamate based binders of the aforesaid U.S. Pat. No. 4,389,341 and U.S. Pat. No. 4,427,579.
However, it has been found that the improved plastic properties provided by such binder systems may not be lasting in that they exhibit a tendency to diminish over prolonged periods of time following their blending with nuclear fuel material. Thus, it is not feasible to store or retain over extended periods molding compositions comprising admixtures of particulate ceramic materials and such amine carbonate or carbamate binders. This shortcoming imposes an impediment to production scheduling and any shipping which entail prolonged periods.
Experience with the amine carbonate or carbamate-types of fugitive binder admixed with uranium dioxide-containing nuclear fuel material indicates that exposure to moisture is a likely factor in diminishing the initially effective plasticity provided by such binders. Moreover, elevated temperatures have also been found to reduce the plastic properties of these admixtures. Thus, over long periods of time, molding combinations of particulate ceramic materials containing uranium dioxide and such amines tend to lose their stability by becoming more brittle and less amenable to rapid compression molding, with the result of a high rate of rejects during compression molding. It appears that it is the presence of carbonate and carbamate ions in such binder structures that lends to unstable chemical and material processing characteristics.